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ApplicationDesignPatterns:Producer/Consumer

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TheProducer/ConsumerdesignpatternisbasedontheMaster/Slavepattern,andisgearedtowardsenhanceddatasharingbetweenmultipleloopsrunningatdifferentrates.AswiththestandardMaster/Slavedesignpattern,theProducer/Consumerpatternisusedtodecoupleprocessesthatproduceandconsumedataatdifferentrates.TheProducer/Consumerpattern’sparallelloopsarebrokendownintotwocategories;thosethatproducedata,andthosethatconsumethedataproduced.Dataqueues(whicharecoveredmorein-depthintheQueuedMessageHandlersection)areusedtocommunicatedatabetweenloopsintheProducer/Consumerdesignpattern.Thesequeuesoffertheadvantageofdatabufferingbetweenproducerandconsumerloops.

TheProducer/Consumerpatterniscommonlyusedwhenacquiringmultiplesetsofdatatobeprocessedinorder.Supposeyouwanttowriteanapplicationthatacceptsdatawhileprocessingthemintheordertheywerereceived.Becausequeuingup(producing)thisdataismuchfasterthantheactualprocessing(consuming),theProducer/Consumerdesignpatternisbestsuitedforthisapplication.Wecouldconceivablyputboththeproducerandconsumerinthesameloopforthisapplication,buttheprocessingqueuewillnotbeabletoaddanyadditionaldatauntilthefirstpieceofdataisdoneprocessing.TheProducer/Consumerpatternapproachtothisapplicationwouldbetoqueuethedataintheproducerloop,andhavetheactualprocessingdoneintheconsumerloop.Thisineffectwillallowtheconsumerlooptoprocessthedataatitsownpace,whileallowingtheproducerlooptoqueueadditionaldataatthesametime.

Thisdesignpatterncanalsobeusedeffectivelywhenyzingnetworkcommunication.

Thistypeofapplicationwouldrequiretwoprocessestooperateatthesametimeandatdifferentspeeds.Thefirstprocesswouldconstantlypollthenetworklineandretrievepackets.Thesecondprocesswouldtakethesepacketsretrievedbythefirstprocessandyzethem.Inthisexample,thefirstprocesswillactastheproducerbecauseitissupplyingdatatothesecondprocesswhichwillactastheconsumer.ThisapplicationwouldbenefitfromtheuseoftheProducer/Consumerdesignpattern.Theparallelproducerandconsumerloopswillhandletheretrievalandysisofdataoffthenetwork,andthequeuedcommunicationbetweenthetwowillallowbufferingofthenetworkpacketsretrieved.Thisbufferingwill

everyimportantwhennetworkcommunicationgetsbusy.Withbuffering,packetscanberetrievedandcommunicatedfasterthantheycanbeyzed.

WhyuseProducer/Consumer?

BuildaProducer/Consumer

Example-Move-Window.vi

ImportantNotes

WhyuseProducer/Consumer?

TheProducer/Consumerpatterngivesyoutheabilitytoeasilyhandlemultipleprocessesatthesametimewhilei tingatindividualrates.Whatmakesthispatternuniqueisitsaddedbenefitofbufferedcommunicationbetweenapplicationprocesses.Whentherearemultipleprocessesrunningatdifferentspeeds,bufferedcommunicationbetweenprocessesisextremelyeffective.Forexample,anapplicationhastwoprocesses.Thefirstprocessperformsdataacquisitionandthesecondprocesstakesthatdataandcesitonanetwork.Thefirstprocessoperatesatthreetimesthespeedasthesecondprocess.IftheProducer/Consumerdesignpatternisusedtoimplementthisapplication,thedataacquisitionprocesswillactastheproducerandthenetworkprocesstheconsumer.Withalargeenoughcommunicationqueue(buffer),thenetworkprocesswillhaveaccesstoalargeamountofthedatathatthedataacquisitionloopacquires.Thisabilitytobufferdatawillminimizedataloss.

BuildaProducer/Consumer

AswiththestandardMaster/Slavepattern,theProducer/Consumerdesignconsistsofparallelloopswhicharebrokendownintotwocategories;producers,andconsumers.Communicationbetweenproducerandconsumerloopsisdonebyusingdataqueues.

LabVIEWhasbuiltinqueuefunctionalityintheformofVIsinthefunctionpalette.TheseVIscanbefoundinthefunctionpaletteunderAdvance>>Synchronization>>QueueOperations.Queuesarebasedonthefirst-in/first-outtheory.IntheProducer/Consumerdesignpattern,queuescanbeinitializedoutsideboththeproducerandconsumerloops.Becausetheproducerloopproducesdatafortheconsumerloop,itwillbeaddingdatatothequeue(addingdatatoaqueueiscalled“enqueue”).Theconsumerloopwillberemoving

datafromthatqueue(removingdatafromaqueueiscalled“dequeue”).Becausequeuesarefirst-in/first-out,thedatawillalwaysbeyzedbytheconsumerinthesameorderastheywerecedintothequeuebytheproducer.Figure1illustrateshowtheProducer/ConsumerdesignpatterncanbecreatedinLabVIEW.

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Figure1:Producer/ConsumerDesignModel

Example-Move-Window.vi

Thisapplicationhasthefollowingrequirements:

Createauserinterfacewithfourdirectionalcontrolbuttonsandaqueuestatusindicator.

Createoneloopthatcollectsuserinterfaceeventsandupdatesthequeueindicator.Createanotherloopthattakestheuserinterfacedataandmovesthewindowaccordingly.

Ourfirststepwillbetodecidewhichloopwillbetheprocessandwhichtheconsumer.Becausetheuserinterfaceiscollectinginstructionaldataforanotherprocesstocarryout,theuserinterfaceloopwillbetheproducer.Theloopthatmovesthewindowdependingonuserinstructionwillbetheconsumer.Theproducerloopwilluseaqueuetobufferuserinterfacedatafortheconsumerloop.Ourapplicationshouldalsomonitorinstructionsthatarecedintoandremovedfromthequeue.

WearenowreadytobeginourLabVIEWProducer/Consumerapplication.ToviewthefinalProducer/Consumerapplication,pleaseopentheattachedVI(Move-Window.vi).

ImportantNotes

TherearesomecaveatstobeawareofwhendealingwiththeProducer/Consumerdesignpatternsuchasqueueuseandsynchronization.

Queueuse

Problem:Queuesareboundtooneparticulardatatype.Thereforeeverydifferentdataitemthatisproducedinaproducerloopneedstobecedintodifferentqueues.Thiscouldbeaproblembecauseofthecomplicationaddedtotheblockdiagram.

Solution:Queuescanacceptdatatypessuchasarrayandcluster.